fiestaturbo.com

[Brian]

I've been looking at the technical data for the 1.4 litre Ford engine, the carb version and the CFi version have different compression ratios, i think 9.5:1 and 8.5:1 respectively. However, every single other specification is the same.

How can this be?

[FezzR]

y does anything else have to be different?

[Brian]

how does the compression ratio change? what does it depend on?

[Richards MGX]

On a fuel injection, the fuel is squirted in under pressure..... so the engine doesn't need to be under as much compression.......

[Richards MGX]

+ the extra performace is achieved by a bigger bang! Higher pressure with the extra perfomance would probably blow the engine.......

[Brian]

So by that thinking, the compression ratio is dependent only on how it receives the fuel, so by changing the inlet manifolds of a CFi and a carb unit, the compression ratio will change??

So, without going into the complicated whys and wherfores or wiring, the inlet manifolds can be swapped, as the compression ratio of the block to which it is attached is only dependent on the inlet manifold, and this will change automatically.

I hope you understand what im asked, can i swap the inlet manifolds...

All other engine specifications are the same

[Richards MGX]

As far as I know, it depends on a few factors......

but that would explain the above......

[Brian]

which few factors

[Richards MGX]

I'm not a mechanic, but bore and stroke affect the pressure too..

...don't they?

Not a mechanic as I said......

[Brian]

thats what i thought, but the bore and stroke are the same for all 1.4 blocks

i'll try it, if theres smoke, a bang or loud noises i know not to do it again!

[FezzR]

no compression ratio is a fixed number

it is the difference between the volume of the cylinder when piston is at bottom dead centre and the small gap left between the pistion and the domed volume of the head when the piston is at top dead centre!

so changing manifolds or injection of fueling will make no difference

[Brian]

In that case, any ideas on how the compression ratios are different on the CFi and carb models?? The carb has greater compression i think.

[FezzR]

of course changing piston size/ bore / or stroke will change the ratio!

so its not completely fixed

[Brian]

but according to Ford specs all the intern dimensions/stroke/bore etc are identical including valve stem tolerances and really silly stuff.

SO there is nothing i can think of that can make the compression ratio change, aprt from fuelling

[James-UK]

Fueling will no effect to compression ratio what so ever.

The 1.4 CFi has different pistons, which is why it has a different compression ratio.

[festajon]

The change in the compression can only be caused by the distance between the pistons and the valves. This can be done by skimming the head and or block, (to raise compression) or by adding distance between the block and the head by using a thicker HG (lowering compression). Another way of lowering compression is skimming the pistons or using special low comp pistons. Also the shape of the piston is designed for efi or carb and as a result the shape can effect compression. However skimming a head and or block wouldnt be able to raise compression that much safely at a guess (from 8.5 to 9.5) so am guessing its different pistons they use.
Where did u get your data from btw?

Cheers
Jon

[festajon]

Fueling will no effect to compression ratio what so ever.

The 1.4 CFi has different pistons, which is why it has a different compression ratio.

Wish i'd read this before i posted DOH

[Brian]

I got the data from the good old Haynes manual!!

Right, so the pistons are a different shape, this doesn't stop me putting a carb inlet manifold on does it? Will it just mean i will need a different carb that will provide different fuelling that is matched for an 8.5:1 compression ratio?

In other words could i swap the inlet manifolds and then get an engine tuner to set up the fuelling??

I think i have nearly got the answer as to whether to replace my engine!!! It's just i've found a good, cheap low-miler, but its CFi and not carb like mine (and my loom!!!) already is, and i dont wanna be changing that.
Cheers people

[festajon]

Swapping from carb to an efi block, but puttin the cab on it will loose some performance for sure as carbs need a higher CR. My mate had a 2i, standard bhp 110bhp in 2i form. He has had a new head mildly ported etc, piper fast road cam, had his bottom end reconned, stuck som twin 40s on it and a zorst,......... it only made 118bhp now if he'd done it to a carbed engine he'd be looking at more like 125ish, and 130 with bigger valves. It will work, though fuel pumps are mounted in the tank for cfi, so you'll have to take the blanking cover off it and put yours on the engine. Not sure about the CPS either as i assume that it has it (tho am not sure as CFi basically is a 'smarter' carb)

But it is pretty much the same block yes.

good luck mate

[PF Dave]

Hmm lots of confusion flying around in here...

AS correctly pointed out the compression ratio will not change by using different carbs or inlet manifold etc. The compression ratio is probably different due to alternative shaped pistons or it could be the shape of the chamber in the head ie lean burn and hemi on the 1600CVH pre and post 86.

With regard to an injection engine needing less than a carbd car - either engine need compression and an increase to either will benefit it (to a point where it will just pink like hell) your friend with the 2i with carbs on etc could of probably got more power form having better headwork carried out or the carbs setup better...my 1600 made 136bhp at best...and by doing the same thing to a "carb" engine as you call it wont make any difference...in effect the engiens are the same to start with!

[festajon]

I agree dave, but just i'm saying the compression will go up in the same proportion and yes there are differences between the engines, i can think of 2 off the top of my head and thats the bare block and head. the compenents contained within each arent interchangeable and compatable, unless u wanna be flippant and say 'yeah but of course u can fit a 2i cam to a xr engine cos they have the same dimensions...' lol but will it run well after? not as well a if it had been fitted with something designed for it in the first place. Tho i think the crank is interchangeable
Yeah u could do some headwork to a 2i head to get good power blah blah blah, but all i meant was in very basic terms, like if u were to skim 3 thou off both heads, put them back onto their respective bottm ends, the xr one will always have more compression cos of the pistons than the fuel injected designed lump being run off a carb.
All he's doin is swappin engines not searching for 0.021bhp and spending loads on it and it will prob be 2 bhp down on its carbed counter part at the very most. I only gave the example of my mate to show the tuned effects of it. It would be like doin all u can to an engine and not swappin the cam for a higher spec one. doin it to a tuned engine will make far more difference than stickin a rally cam in a boggo xr2. basic tuning rules, which i know u know, but seem to assume i dont cos i dont have a highly tuned car where i'd rather buy a faster car from the beginning and tune that (like an Audi S3 ) and not some car i have to spend the same amount on to get it as fast, handle as well and look even slightly as good as that does, and still have people say 'yeah but its a fiesta'
Goin by ur theory, if i were to fit a 'turbo' engine in to an xr2i, and use all 2i running gear, bar the engine, i would 'hardly notice the difference' with a bit of a skim to get back the lost compression right? they are 'in effect' the same engine after all

*edited cos Al got upset i used the term ceterus paribus and he didnt undestand llama
**furthermore its late, this post is 'tit for tat' and meant as banter so please don't listen to owt i say cos im knackered and grumpy cos i cant sleep

[Project]

FestaJon, you don't half talk some steaming natural fertiliser sometimes! pimsle

[Simmo]

???????

go read http://www.pumaracing.co.uk .... theres loads of info on compression ratios on there, and that guy knows what hes talking about.... proven